1. Field of the Invention
This invention relates to an artificial blood vessel composed of polytetrafluoroethylene, and more specifically, to a vascular prosthesis of a tubular fibrous construction whose inner surface consists of finer fibers than its outer surface, which is expected to expedite the healing of the anastomoses and development of the neointima after a surgical implant operation.
2. Description of the Prior Art
Fabric prostheses composed of a knitted or woven fabric of Dacron or polytetrafluoroethylene having inner diameters that are relatively large are now being utilized, with relatively good results. In particular, good results are generally obtained with vascular prostheses for arteries which have an inner diameter of at least about 7 mm. Despite this, few small inner diameter arteries are clinically acceptable. In venous applications, small inner diameter prostheses show a lower patency rate than in arterial application. The rate of blood flow in veins is smaller than in arteries, and to prevent thrombosis, it is important to inhibit platelet adhesion to the inner surface of the artificial veins. This requirement is not fully met by presently available artificial veins.
Some tubings made of stretched or expanded polytetrafluoroethylene have been demonstrated to be clinically useful as vascular prostheses for arteries and veins. This is described, for example, in Soyer et al., "A New Venous Prosthesis", Surgery, Vol. 72, pages 864 (1972), Volder et al., "A-V Shunts Created in New Ways", Trans. Amer. Soc. Artif. Int. Organs, Vol. 19, p. 38 (1973), Matsumoto et al., "A New Vascular Prosthesis for a Small Caliber Artery", Surgery, Vol. 74, p. 519 (1973), "Application of Expanded Polytetrafluoroethylene to Artificial Vessels", Artificial Organs, Vol. 1, p. 44 (1972), Ibid., Vol. 2, p. 262 (1973), and Ibid., Vol. 3, p. 337 (1974), Fujiwara et al., "Use of Goretex Grafts for Replacement of the Superior and Inferior Venae Canal", The Journal of Thoracic and Cardiovascular Surgery, Vol. 67, p. 774 (1974), and Goldfarb, Belgian Pat. No. 517,415.
The results of these clinical experiments are summarized below.
When a suitable porous prosthesis is implanted as a conduit within the arterial system, the fine pores are clogged by clotted blood, and the inside of the prosthesis is covered with the clotted blood layer. The clotted blood layer is made up of fibrin, and its thickness varies according, for example, to the material of the prosthesis, and the surface structure of the prosthesis. Since the thickness of fibrin approaches 0.5 to 1 mm when a knitted or woven fabric of Dacron or polytetrafluoroethylene is used as the prosthesis, success is achieved only with those blood vessels which are not occluded by this increase in the wall thickness by the fibrin layer (that is, arteries having an inside diameter of 5 to 6 mm or more). Generally, knitted or woven prosthesis having smaller inner diameters have not been successful.
A polytetrafluoroethylene tubing which has been stretched has a microstructure composed of very fine fibers and nodes connected together by the fibers. The diameters of the fibers vary depending on various stretching conditions, and can be made much smaller than fibers of the knitted and woven fabrics mentioned above.
It has been confirmed clinically that when a structure composed of fibers and nodes is expressed in terms of pore sizes and porosities, or fiber lengths and nodular sizes, a polytetrafluoroethylene tubing having a pore size of from about 2.mu. to about 30.mu. (pore sizes below about 2.mu. are undesirable), a porosity of about 78% to about 92%, a fiber length of not more than about 34.mu. (fiber lengths of about 40.mu. to about 110.mu. are undesirable), a nodular size of not more than about 20.mu., and a wall thickness of about 0.3 mm to about 1 mm exhibits a high patency rate without substantial occlusion by fibrin deposition.
It has been reported, however, that venous prosthesis shows a much lower patency rate than arterial prosthesis, and does not prove to be entirely satisfactory for prosthetic purposes. It has also been reported that when the vascular prosthesis has too high a porosity, a tearing of the prosthesis by the suture used in joining the prosthesis with the vessel of the patient tends to occur.